The present invention is directed to a method of enhancing the contrast of images of objects such as masks for photolithography in the manufacture of integrated circuits and materials therefor.
Lithography in the production of integrated circuits is predominantly carried out by optical means. In the drive to reduce circuit dimensions, improved performance and increase yield, optical systems have provided the required resolution with each successive generation of circuit technology. The image resolution of projection lithographic systems has recently begun to approach the physical limits imposed by practical constraints on numerical aperture and wavelength. While further improvements in lithographic technology are anticipated, dramatic improvements in inherent lens resolution are not. In order to continue the reduction of minimum feature size achievable by optical techniques, it is necessary to alter some other aspect of the lithographic process for further improvements. One area in which further improvements are possible is in the photoresist process. Each photoresist is characterized by some degree of incident contrast necessary to produce patterns usable for subsequent processing. This minimum required contrast of illumination is referred to as the contrast threshold of the resist. Depending on substrate properties, the required pattern thickness and resist edge profiles, conventionally used positive photoresist has a contrast threshold between 85% and 90% contrast. Currently, most production is done at 90% incident contrast or more. If the contrast threshold of the resist is reduced, the resolution obtainable with a given optical system is improved due to the fact that image contrast is a decreasing function of the spatial frequencies present in the image.
The present invention is directed to provide a photoresist process in which the contrast of an aerial image utilized in the process is enhanced prior to incidence on the photoresist.
An object of the present invention is to lower the minimum contrast required to produce usable images in a photoresist.
Another object of the present invention is to provide new photobleachable compounds and materials.
In carrying out the invention in an illustrative embodiment thereof, a layer of photoresist of a first thickness and having a predetermined contrast threshold is provided. An object or mask is provided having opaque and transparent areas. An image having a contrast less than the predetermined contrast threshold of the photoresist is formed of the object on the layer of photoresist by projecting light of a predetermined wavelength through the object. A layer of photobleachable material including a photobleachable compound is provided between the object and the layer of photoresist and adjacent a surface of the layer of photoresist. The photobleachable compound is sensitive to the aforementioned wavelength of light and has an extinction coefficient to molecular weight ratio in liters per gram-centimeter in the unbleached state greater than about 10. The ratio of the extinction coefficient for the unbleached state to the extinction coefficient for the bleached state of the photobleachable material is also greater than about 10. Light of the aforementioned wavelength and of a predetermined intensity is projected through the object onto the layer of photobleachable material for a time to obtain a reduction in optical density of the layer of photobleachable material in direct proportion to the dosage of light of the aforementioned wavelength incident thereof, whereby the integrated contrast of the image transmitted by the layer of photobleachable material increases with dosage transmitted thereby, reaches a maximum value and thereafter decreases. Parameters of the layer of photobleachable material are selected such that the maximum value of integrated contrast is greater than the predetermined threshold contrast of the layer of photoresist. The sensitivity of the layer of photoresist and the thickness thereof are selected such that the layer of photoresist is fully exposed by a dosage in a predetermined range transmitted by the layer of photobleachable material and provides an integrated contrast in the transmitted image above the predetermined contrast threshold of the photoresist. Light of the aforementioned predetermined wavelength is projected through the object for a time to provide the dosage in aforementioned predetermined range transmitted through the photobleachable layer. The layer of photobleachable material is removed and the layer of photoresist is developed whereby a pattern representing an enhancement in contrast of the image of reduced contrast of the object is formed in the layer of photoresist.
Another aspect of the present invention is directed to a spin castable mixture capable of forming a photobleachable layer having an absorption maximum in the range of 300 to 450 nanometers comprising by weight
(A) 100 parts of organic solvent, PA0 (B) 1 to 30 parts, preferably 5 to 15 parts, of an inert organic polymer binder, PA0 (C) 1 to 30 parts, preferably 5 to 15 parts, of an aryl nitrone.